Fix flaky ServerSetDiscoveryTest (#6481)

Motivation:

#6451 #6408 
I'm not 100% sure but test failures seem to have decreased after
increasing the timeout.

Modifications:

- Wait up to 60 seconds for the zookeeper test server to start.

Result:

- Closes #6451
- Closes #6408

Author: ikhoon

zookeeper3/src/test/java/com/linecorp/armeria/common/zookeeper/ZooKeeperExtension.java

@@ -33,7 +33,7 @@
 
 public class ZooKeeperExtension extends AbstractAllOrEachExtension implements ZooKeeperAssert {
 
-    private static final Duration duration = Duration.ofSeconds(10);
+    private static final Duration duration = Duration.ofSeconds(60);
     private static final TemporaryFolder tempFolder = new TemporaryFolder();
 
     @Nullable